No, one astronomer instead thinks that our Solar System may once have had a fifth giant planet – and thus a 10th major planet – four billion years ago. This planet would have been a Neptune-mass planet that lived between Saturn and Uranus until, for some unknown reason, it was ejected from the Solar System. So long.
This is the theory proposed by David Nesvorny from the Southwest Research Institute in Boulder, Colorado, in The Astronomical Journal, and reported by Science Magazine and New Scientist. He first proposed his theory back in 2011, but has since revised it based on new data.
The key piece of evidence comes from a cluster of about 25,000 icy rocks called the “kernel” that reside in the Kuiper Belt, the region at the edge of the Solar System. While most of the trillion or so objects in the Kuiper Belt are scattered around the Solar System, the kernel orbits in the same plane as the planets. Until now, no one really knew why.
But modeling the Solar System, Nesvorny was able to “rewind” and take a look at where these rocks came from. He found that they were once under the influence of Neptune’s gravity, albeit further from the Sun; Neptune orbited at a distance of 4.2 billion kilometers (2.6 billion miles), and the kernel at 6.9 billion kilometers (4.3 billion miles). But suddenly, four billion years ago, Neptune’s orbit was shifted and it moved 7.5 million kilometers (4.7 million miles) outwards. The kernel, unable to keep up, escaped the clutches of Neptune, and remained in the same position, where it is today.
“The Kuiper Belt is the clue,” Nesvorny told Science Magazine. “You see the structures there, and you try to figure out what kind of evolution would fit those structures.”
The only plausible explanation for this jolt, Nesvorny surmises, must have been a rogue object passing by, and the best explanation is a fifth gas giant. The other planets in the Solar System were ruled out as they could not interact with Neptune in such a way, as were up to 100 other possibilities.
What became of this planet is not known. Nesvorny thinks it may have been ejected by the other planets in the Solar System, changing the orbit of Neptune in the process, but other than being similar to Neptune in mass – deduced from how Neptune’s orbit changed – nothing else can be inferred about it.
It’s certainly an interesting theory, although of course much more research will be needed to confirm or deny it. Planets migrating through the Solar System is not an unchartered area, either; some scientists think Jupiter swung through the inner Solar System early in its life like a "wrecking ball", sweeping out some debris and giving rise to the inner rocky planets like Earth.
Whatever the case, continued observations could reveal that our Solar System was once not the relatively more sedate place we see today.